Electrophotographic (“EP”) printing involves transferring toner, or dry ink, to a substrate, such as paper, by means of an electric field and then fusing the toner to the substrate using a combination of heat and pressure. After fusing, the substrate is cooled, and excess charge is removed from the substrate. Conventionally, a release fluid is used during the fusing process to provide release of the substrate from the fusing roller. After fusing, cooling, and removing excess charge, the substrate exits the EP printing device, thereby completing the printing process. The substrate having an image fused thereon by an EP printing process is referred to as a “printed document” and may contain text, one or more images, or both.
Commonly, the printed document subsequently is subjected to a finishing procedure. Examples of finishing procedures include glossing, coating using ultraviolet (“UV”) radiation, and lamination. In the case of glossing, the printed document is subjected to a procedure that heats and casts the fused toner on the printed document to give it a glossy appearance. In the case of coating using UV radiation, the printed document is coated with a UV curable fluid and exposed to such UV radiation. In the case of lamination, a coating, such as plastic, is applied to the printed document and is heated under pressure to form a protective coating over the printed document.
During each of these finishing procedures, performance and/or quality problems arise if there is a significant amount of release fluid remaining on the printed document when the finishing procedure is performed. These problems will be described in more detail with reference to FIG. 1. Illustration 101 shows an arrangement of toner particles 102 on a substrate 103 prior to being fused. Illustration 104 shows toner particles 105 that have been over-fused to the substrate 103. In particular, the toner particles 105 have been fused to form a mostly continuous layer. In this case, the release fluid 106 cannot migrate into the substrate 103. Consequently, the release fluid 106 sits on top of the over-fused toner particles 105 and becomes a problem for downstream processes, such as subsequent finishing procedures.
For example, if a glossing procedure is applied to the over-fused printed document illustrated at 104, the release fluid will interact with the polishing device in the glossing apparatus, thereby degrading performance. If a UV coating is applied to the substrate 103 having the over-fused toner 105 and release fluid 106 thereon, as illustrated at 104, the UV curable material may not adequately coat the image thereby resulting in image quality artifacts and non-uniform image protection. If a laminate coating is applied on top of the over-fused toner 105, the laminate forms on top of the release fluid 106 causing artifacts, such as rivers or lakes, or poor adhesion of the laminate to the image.
Illustration 107 shows properly-fused toner particles 108 that, although adhered to the substrate 103, have seams 109 between them, that allow release fluid (not shown) to migrate into the substrate 103. Accordingly, the release fluid (not shown in illustration 107) does not sit on top of the properly-fused toner particles 108 and does not become a problem for downstream finishing processes.
It has been difficult conventionally to ensure that proper-fusing of toner particles as shown in illustration 107 occurs for subsequent finishing procedures, particularly because proper-fusing is dependent upon many variables. Accordingly, a need in the art exists for an optimized printing process that reliably provides proper-fusing for subsequent finishing procedures.